How to call the StoredProcedure with Linq that has Output parameters

CREATE PROCEDURE [dbo].[InsertCustomer]
(
	
	@LastName varchar(50), 
	@FirstName varchar(50), 
	@Address varchar(50), 
	@City varchar(50), 
	@State char(2), 
	@ZIP varchar(10),
	@CustomerID int OUTPUT,
	@Modified timestamp OUTPUT
)
AS
 SET NOCOUNT ON;
 
 -- Inserts a Customer row
 INSERT INTO Customer 
	( LastName, FirstName, Address, City, State, ZIP ) 
 VALUES 
	( @LastName, @FirstName, @Address, @City, @State, @ZIP ) ;
 
 IF @@ROWCOUNT>0 AND @@ERROR=0 -- Checks if the last statement produced an error
	-- Selects the primay key value and the generated timestamp which is put back into the dataset
   SELECT @CustomerID = CustomerID, 
		  @Modified = Modified 
   FROM Customer 
   WHERE (CustomerID = SCOPE_IDENTITY());
   RETURN

System.Nullable<int> CustomerId = 0;
            System.Data.Linq.Binary lastChanged = null;
            try
            {
                using (OMSDbDataContext ctx = new OMSDbDataContext())
                {
                    ctx.InsertCustomer(
                    TxtLastName.Text.ToString(),
                    TxtFirstName.Text.ToString(),
                    TxtAddress.Text.ToString(),
                    TxtCity.Text.ToString(),
                    TxtState.Text.ToString(),
                    TxtZip.Text.ToString(),
                    ref CustomerId,
                    ref lastChanged);
                }

            }
            catch (Exception ex)
            {

                throw new Exception(ex.Message);
            }

            finally
            {
                this.ClientScript.RegisterStartupScript(this.GetType(), "Success", "alert('Record Inserted with CustomerId = " + CustomerId + "');", true);
            }